Question

PLS HELP I'M SOO CONFUSED

Answer 1

The first term in the series is 336. The second term is obtained by subtracting 4 from the first term; subtract 4 from that to get the third term; and so on. Then the n-th term in the series is

336 - 4 (n - 1)

or

340 - 4n

The last term in this series is 4, so we solve for n :

340 - 4n = 4

336 = 4n

n = 336/4 = 84

The sum of the series is then

$\displaystyle\sum_{n=1}^{84}(340-4n) = 340\sum_{n=1}^{84}1 - 4 \sum_{n=1}^{84}n$

Recall that

$\displaystyle \sum_{n=1}^{N} 1 = N$

$\displaystyle \sum_{n=1}^N n = \frac{N(N+1)}2$

Then the sum we want is

$\displaystyle\sum_{n=1}^{84}(340-4n) = 340\times84 - 4\times\frac{84\times85}2 = \boxed{14,280}$